1. Field of the Invention
The present invention relates to an electronic system that enables distinguishing between sinus and nonsinus atrial depolarizations which, when incorporated into cardiac pacemakers, prevents the pacemakers from stimulating ventricular contractions following nonsinus atrial depolarizations.
2. Description of Related Art
Contraction of cardiac muscle, like all muscle tissue, occurs in response to changes in the electrical potential of the membranes of the muscle cells. When the changes in membrane electrical potentials are recorded and plotted against time, a pattern characteristic of sinus cardiac depolarization is produced. First, a P wave corresponding to depolarization of the atria is seen. A fraction of a second later, a QRS complex corresponding to ventricular activation is observed.
In normal sinus cardiac activation, the wave of electrical depolarization begins in the high right atrium in the sinoatrial (SA) node. From there the wave spreads over the atria in a pattern resembling spreading of waves created by throwing a pebble into water. Located near the juncture of the right atrium and right ventricle is the atrioventricular (AV) node. The wave of depolarization that has spread over the atria converges at the AV node and from there is conducted via specialized conduction fibers to the ventricles resulting in ventricular activation. The delay inherent in conduction from the atria to the ventricles allows sufficient time for atrial contraction to fill the ventricles, and thus maximizes the efficiency of each ventricular contraction.
Because the sequence of atrial and ventricular depolarization observed in normal sinus rhythm optimizes blood flow with each ventricular contraction, maintenance of this sequence is required for continued good health. In a number of disease states, however, normal sinus rhythm is disrupted. For example, when premature ventricular contractions occur, the wave of depolarization begins in a ventricle causing the ventricles to contract before they have been filled with blood from the atrial contraction. If premature ventricular contractions occur too frequently, the reduced amount of blood pumped can be life threatening. Another common abnormal cardiac rhythm is seen when normal conduction from the atria to the ventricles is partially or totally blocked. In these patients, the wave of depolarization begins in the SA node as in normal sinus rhythm, but the wave is not conducted to the ventricles and thus fails to result in the blood being pumped through the pulmonary and systemic vascular systems.
Electronic pacemakers have proved a very effective and reliable treatment modality in patients whose normal sinus rhythms have been disrupted by disease. Many pacemakers in current use that are implanted are dual chambered devices that are capable of sensing and pacing in the atrium and the ventricle. These pacemakers are called "universal" since they can perform almost any function. When the patient's atrial rate falls below a predetermined level, the pacemaker paces the atrium. After a delay sufficient to permit conduction from the atrium to the ventricle, if conduction has not occurred, the pacemaker also paces the ventricle. In many cases the atrial rate is sufficient, however, there is a lack of appropriate AV conduction. In these patients, the pacemaker will sense the patient's own atrial P wave and then pace the ventricle after the P wave.
One of the greatest difficulties in modern pacemaking is that current pacemakers cannot distinguish between sinus P waves and nonsinus P waves. Two major types of nonsinus P waves exist. The first is the "retrograde" P wave. This can occur any time the ventricle is paced by a pacemaker or a premature ventricular contraction occurs with conduction of the ventricular impulse retrogradely to the atrium (retrograde conduction can exist even in a patient with complete antegrade AV block). In this instance, the pacemaker senses the retrograde P wave and responds as though it were a normal sinus P wave. Since the pacemaker is programmed to pace the ventricle after every sensed P wave, an "endless loop" type of pacemaker mediated tachycardia can result. Every time the pacemaker paces the ventricle, it conducts retrogradely to the atrium, is sensed by the pacemaker, and then the pacemaker undesirably paces the ventricle.
Many artificial adjustments are made in current pacemakers in an attempt to circumvent "endless loop" pacemaker mediated tachycardia. The majority of these attempts involve creation of a time during the cardiac cycle when the pacemaker does not sense in the atrium. U.S. Pat. No. 4,343,311 provides an example of this approach. The disadvantages occasioned in creating a time when the pacemaker does not sense in the atrium include patient-to-patient variability in the time delay required, the lack of atrial sensing throughout the entire cardiac cycle, and restricting the upper limit that the pacemaker can pace the ventricle.
The second type of nonsinus P wave is observed in supraventricular tachycardia. In the vast majority of cases of supraventricular tachycardia, the pacemaker senses the P waves and will pace the ventricle at the rate of the P wave. The normal human AV node acts as a filter such that, in the adult, atrial impulses faster than 200 per minute usually will not be transmitted through the AV node to the ventricles. This is a natural event that a pacemaker can circumvent. For this reason, pacemakers have an "upper rate limit" where, regardless of atrial rate, the pacemaker will not transmit impulses to the ventricle above this rate. U.S. Pat No. 4,059,116 discloses a pacemaker that employs an upper rate limit. If supraventricular tachycardia could be distinguished from a normal rapid sinus tachycardia, the artificial "upper rate limit" could be eliminated.
In "retrograde" P waves, the sequence of atrial activiation differs from sinus P waves. In sinus P waves, the atria are activated from the SA node and activation spreads from the high atrium to the low atrium. In "retrograde" P waves, the atria are activated in the reverse direction from low atrium to high atrium. In over 90 percent of supraventricular tachycardias, the atria are activated in a different sequence from sinus rhythm. Therefore, sinus and nonsinus P waves can be distinguished by determining the atrial activation sequence.
In currently used pacemakers, sensing of the atrium is either "unipolar" or "bipolar." In unipolar sensing, there is a single electrode at the tip of the pacemaker lead. This lead usually is placed in the right atrial appendage (RAA) and uses the pacemaker generator as the indifferent electrode. In the bipolar configuration two electrodes are placed approximately 20 mm apart at the end of the lead which also is placed in the right atrial appendage. None of the pacemakers in current use possess a design that enables determining the atrial activation sequence to distinguish between sinus and nonsinus P waves.